In data transmission networks, it is often necessary to protect stored and transmitted data from intelligent interception by unauthorized parties known as eavesdroppers. Cryptography generally offers the protection necessary for data privacy. However, for stream cipher operation in which incoming data characters are not treated independently, certain types of cryptographic systems afford more privacy than others. For this reason, private key cryptographic systems rather than public key cryptographic systems have been employed for directly generating a private stream cipher.
In stream cipher operation, every incoming data character is enciphered into an output data character on the basis of an internal state of the encryption system. After each character is enciphered with a secret key, the encryption system changes state in accordance with a prescribed criterion. Hence, two occurrences of the same incoming data character generally produce different enciphered output data characters.
Privacy of stream cipher operation resides in the fact that an eavesdropper cannot recover the original incoming data characters from a received set of characters without knowledge of the secret key employed during encryption. If the secret key becomes known, the privacy of the cryptographic system is compromised. Since the encryption process and the encryption key are both publicly known in a public key cryptographic system, privacy is nonexistent when the public key cryptographic system directly generates stream ciphers.